This invention relates generally to electronic identification systems which provide the means for the cooperative identification of objects by means of tags attached to or imbedded in the objects. More specifically, the invention relates to identification tags which can cooperate with a variety of types of tag readers.
A key element of an electronic identification system is a means for communication between reader and tag and, since a tag usually has no independent source of power, a means of transferring power from reader to tag in a sufficient amount to permit the tag to perform its functions. These two functions can both be accomplished with electromagnetically-coupled readers and tags, the predominant technology presently in use. The reader establishes an alternating magnetic field in the vicinity of a tag and the tag extracts both information and power from the magnetic field. The efficient extraction of information and power from an alternating magnetic field mandates the use of a resonant circuit of some sort by the tag.
The first close-coupled electronic identification systems, i.e. reader and tag in close proximity when communicating, consisted of readers which transmitted unmodulated carriers and tags which respond with signals that carried data. System capabilities have been extended in recent years with readers that use modulated carriers to transmit data to tags.
The carrier frequencies used by electronic identification systems of the close-coupled variety have ranged from 100 kHz to 2 GHz in the past. Recent efforts at standardization point to a frequency in the 110 to 135 kHz range as being appropriate for worldwide use.
The most significant difference in present-day close-coupled systems is whether the reader is or is not transmitting the carrier when the tag responds with data. Systems in which the reader transmits during the tag response are called xe2x80x9cfull-duplexxe2x80x9d (FDX). Systems in which the reader is silent during the tag response are called xe2x80x9chalf duplexxe2x80x9d (HDX). In an HDX system the reader transmission periods are interlaced with the tag response periods so as to minimize the energy storage requirements in the tag.
A tag transmits data to a reader by modulating a carrier. The frequency of the tag""s carrier can be the same as or different from the frequency of the reader""s carrier. When the frequencies of the reader""s carrier and the tag""s carrier are the same, it may seem in some tag designs that the tag is not using a carrier. Instead, the tag is simply modulating the reader""s carrier by absorbing more or less energy as a function of time from the alternating magnetic field established by the reader. A better understanding of the communication principles can be had, however, if the details of tag design are ignored in favor of the more general view that the tag creates a modulated carrier with a frequency the same as or different from the frequency of the reader""s carrier. The tag""s carrier produces a separate alternating magnetic field which is superimposed on the alternating magnetic field established by the reader.
There are a variety of ways in which the reader and tag can modulate their respective carriers with data. One can start with amplitude shift keying (ASK), phase shift keying (PSK), and frequency shift keying (FSK), the names of which indicate the carrier parameter that is modulated. These modulation types are typically used in binary versions wherein the parameter can take on either one of two values. It may become desirable in the future to use n-level forms of these modulation types in order to realize certain communication efficiencies.
The next level of modulation complexity is to combine these basic types of modulation in a variety of ways as, for example PSK/FSK wherein both the phase and the frequency of a carrier carries data.
A different way of combining modulation types is to piggyback one modulation type on another as, for example, when a subcarrier is frequency shift keyed in accordance with the bits in a message, and then the carrier is amplitude modulated by the FSKed subcarrier.
The communications between reader and tag are in the form of messages consisting of a finite number of bits. Each message bit is usually translated into one or more transmit bits prior to modulating a carrier. The typical translations include (besides the identity translation where the message bits are also the transmit bits):
Manchesterxe2x80x940 translates into 01 translates into 10;
Millerxe2x80x94T(N,1)=T(N-1,2) EX.OR [Mbar(N-1) AND Mbar(N)]
T(N,2)=T(N,1) EX.OR M(N)
where M(N) is the N""th message bit, Mbar(N) is M(N) inverted, and T(N,1), T(N,2) are the first and second transmit bits associated with the N""th message bit.
Electronic identification systems which utilize implantable or attachable tags have proliferated over the past decade to the point where users are seriously inconvenienced by the incompatible equipments produced by vendors who participate in this market. In general, tags supplied by one vendor cannot be read by the readers supplied by another vendor which means that users necessarily find themselves locked into the systems of one manufacturer. For large-scale applications of electronic identification to occur, some means for assuring equipment compatibility is essential.
There are a number of avenues that can be followed in achieving equipment compatibility. The typical approach to achieving interoperability of equipments is the establishment of standards for this purpose. The establishment of standards has the disadvantage of tending to freeze technology and hinder the development of more advanced systems.
Another approach is to make available xe2x80x9cuniversalxe2x80x9d tag readers which can read the tags that are presently being used, and which can be economically upgraded to reading tags that are developed in the future.
A third approach is to make available xe2x80x9cuniversalxe2x80x9d tags which can be read by any reader that is presently being used, and which can be upgraded for use with readers of different designs that appear in the future.
The universal electronic identification tag is for use with a variety of readers of different designs including a control reader which can be used to control the operations of the universal tag. A reader interrogates a tag by transmitting a carrier.
The universal tag comprises a transducer, a modulator connected across the transducer, and a control means. The control means causes the modulator to drive the transducer with a plurality of different message waveforms after interrogation and while the carrier is present and also after interrogation and while the carrier is absent, the tag identity being embedded in each of the message waveforms.
The presence of a carrier is determined by an alerting device which generates an alerting signal having a first value when a reader carrier is less than a predetermined magnitude and a second value when the reader carrier is greater than a predetermined magnitude. The control means causes the modulator to drive the transducer with a plurality of different message waveforms after the value of the alerting signal changes from the first value to the second value, the tag identity being embedded in each of the message waveforms. The control means also causes the modulator to drive the transducer with one or more different message waveforms when the value of the alerting signal changes from the second value to the first value, again with the tag identity being embedded in each of the message waveforms.
A message waveform is comprised of a sequence of contiguous waveform segments, each waveform segment representing the value of an N-bit group, N being an integer. The waveform segments used to represent the values of an N-bit group in at least one message waveform are different from those used to represent an N-bit group in the other message waveforms.
A waveform segment is a periodic signal characterized by the parameters frequency, phase, and amplitude. The waveform segments which represent the values of an N-bit group are differentiated by the values of at least one parameter.
The control means causes the modulator to drive the transducer either simultaneously, sequentially, or simultaneously and sequentially with a plurality of message waveforms.
The modulator comprises a plurality of driving circuits connected across the transducer. The control means causes the modulator to drive the transducer with one or more message waveforms using one of the driving circuits after interrogation by a reader. At least one of the driving circuits includes a resistive load across the transducer, the magnitude of the resistive load being determined by the message waveform. Another of the driving circuits injects charge into the transducer, the magnitude of the injected charge being determined by the message waveform. Still another of the driving circuits includes a reactive load across the transducer, the magnitude of the reactive load being determined by the message waveform.
At least one of the driving circuits includes a first load and a second load connected in series across the transducer, the first load being short-circuited for one polarity of the voltage across the transducer, the second load being short-circuited for the other polarity. The control means can be programmed so that only one of the loads is driven by a message waveform. Other programming options provide for the first and second loads to be driven in phase or out of phase.
At least one of the driving circuits includes a first charge injector and a second charge injector connected in series across the transducer, the first charge injector being short-circuited for one polarity of the voltage across the transducer, the second charge injector being short-circuited for the other polarity of the voltage across the transducer. Here also, the control means can be so that only one of the charge injectors is driven, the two charge injectors are driven in phase, or the two charge injectors are driven out of phase.
Since the universal tag must operate with readers which transmit carriers with different frequencies, the control means is programmed to set the resonant frequency of the transducer to the frequency of the carrier being transmitted by a reader.
A two-stage power developer connected across the transducer supplies power to the components comprising the tag, the power developer obtaining power from the voltage induced in the transducer by a reader""s carrier, a first portion of the power supplied by the reader""s carrier being supplied by the power developer directly to the tag components, a second portion of the power supplied by the reader""s carrier being stored by the power developer and supplied to the tag components when the first portion is insufficient to power the tag.
The power developer includes a voltage regulator for powering voltage-sensitive elements of the tag.
An alternative power developer utilizes a battery for supplying power to the components comprising a tag. The power developer recharges the battery with power from the voltage induced in the transducer by a reader""s carrier.
A clock generator connected across the transducer supplies clock signals to the components comprising the tag. The clock generator includes an oscillator locked to the frequency of the voltage induced in the transducer by a reader""s carrier. The clock generator includes a frequency memory which causes the frequency of the oscillator to be maintained at the frequency of the voltage induced in the transducer after the induced voltage disappears.
A digital-to-analog converter, which converts a number supplied by the control means to a voltage, supplies voltages to tag units which require voltages other than those provided by the power developer.
A demodulator connected across the transducer determines whether the voltage appearing across the transducer is unmodulated or modulated with data and supplies the control means with a modulation indicator. If the voltage is modulated, the demodulator extracts and supplies the data to the control means.
The control means executes any commands contained in the extracted data. For example, the control means causes the modulator to drive the transducer with a predetermined one or more of the plurality of message waveforms if the transducer voltage is unmodulated. If, however, the transducer voltage is modulated and the proper command is contained in the extracted data, the control means causes the modulator to drive the transducer with one or more of the plurality of message waveforms specified by the extracted data.
The universal tag includes an EEPROM and an EEPROM programmer the control means, in response to a command contained in the extracted data, modifies its behavior by causing the EEPROM programmer to replace data contained in the EEPROM with data contained in the extracted data.
The control means appends an auxiliary message waveform to the message waveform intended for the control reader, when the control means causes the modulator to drive the transducer with one or more message waveforms after an interrogation by a reader. The auxiliary message waveform is derived from an auxiliary message constructed by the control means.
A sensor circuit having an output which is an uncalibrated measure of an environmental parameter can be incorporated in the universal tag. A memory for storing sensor circuit calibration data is also provided. The control means constructs the auxiliary message intended for the control reader from the sensor output and the sensor circuit calibration data stored in the memory.
The sensor circuit comprises a sensor having an output which is an analog measure of an environmental parameter and an analog-to-digital converter which converts the sensor output to a digital number. One version of the analog-to-digital converter comprises an oscillator having a frequency determined by the magnitude of the sensor output and a counter which counts the number of cycles of the oscillator output signal in a predetermined period of time, the cycle count being a measure of the frequency of the oscillator and the magnitude of the environmental parameter.
The control means utilizes the auxiliary message communication capability to send status data to the control reader. The control means accumulates status data and stores the data in the memory. The control means constructs the auxiliary message from the status data stored in the memory.